The present invention relates to a method and an apparatus for writing clock data to a storage medium.
Information for systems such as data processing systems is typically stored on storage media. Particular use is made of storage disks such as magnetic disks, opto-magnetic disks, and the like. One type of magnetic disk arrangement is a so-called xe2x80x9chead disk assemblyxe2x80x9d which is intended normally to be permanently fixed in a data processing system; the head disk assembly includes the magnetic disk medium itself and the associated read and write head or heads (the xe2x80x9cproductxe2x80x9d head or heads) which write data to the disk and read data from the disk. Another type of magnetic disk is of a type known as xe2x80x9cremovable mediaxe2x80x9d which normally consists of a magnetic disk medium in a protective plastics case which can be used to transfer data between data processing systems by physical transfer of the disk itself from one machine to another.
In a known method of manufacturing storage media such as hard disk drives, a head disk assembly consisting of the product head(s), the disk or disks, the motor and arm electronics, is mounted in a mastering station known as a servo-writer. The servo-writer writes a pattern of magnetic information (the xe2x80x9cservo track patternxe2x80x9d) onto the disk. The servo track pattern becomes the master reference which is used by the disk drive during normal operation in order to locate the tracks and sectors on the disk for data storage and retrieval. Clearly, the servo track pattern has to be accurately written to the disk at very well defined positions.
In order to be able to write the servo pattern onto the correct positions on the disk during manufacture of the disk, in one commonly used process, a (usually temporary) xe2x80x9cclockxe2x80x9d track is written onto the disk to serve as a timing reference during writing of the servo pattern. Conventionally, a separate clock head is used to write the clock track onto the disk and to read the clock track from the disk so that the servo tracks, which are written with the disk drive""s product head, can be phase aligned with respect to the clock track and each other. However, the use of a dedicated clock track writing head is an expensive addition to the manufacturing process and further requires that the servo tracks be written in a clean room because the clock track writing head has to be inserted into the open (unsealed) head disk assembly. Moreover, in practice the clock heads have to be replaced on a daily basis because of damage which occurs during use.
There have been several proposals for methods and apparatus for writing the servo tracks to a disk without the need for a dedicated clock head. Typically, and put briefly, an internal write head of a head disk assembly is used to write an initial clock track around the disk. That initial clock track is then read by an internal read head of the head disk assembly and the next clock track is written by the internal write head, with reading of the previous clock track being interleaved with writing of the present clock track. The servo patterns or tracks are written at appropriate locations on the disk, interleaved with the clock tracks, the positions of the servo patterns or tracks being determined in accordance with timing information obtained from the clock tracks. Such methods, or similar methods, are disclosed in for example U.S. Pat. No. 5,485,322, U.S. Pat. No. 5,448,429, and U.S. Pat. No. 5,668,679.
In our WO-A-98/31015, the entire disclosure of which is incorporated herein by reference, there is disclosed another method and apparatus for writing clock data to a storage medium, such as a disk. The storage medium has tracks on which data can be stored. Clock data is written to a present track on the storage medium. The written clock data is read from that track. Clock data for a subsequent track on the storage medium is generated from the clock data read from said present track. The phase of the generated clock data for the subsequent track is compared with a reference timing signal and the phase of the generated clock data for the subsequent track is adjusted in accordance with said comparison. The phase adjusted clock data for the subsequent track is then written to said subsequent track. In this method and apparatus, it is again not necessary to provide a dedicated clock read/write head. As in other proposals of this type, the servo information is written to the disk using the clock track data to position the servo information very accurately. To save time, it is preferred that the servo information in a track be written alternately with the clock data in that track, i.e. for any track, a portion of clock data is written, that clock data being phase aligned with the clock data in the previous track, and then a burst of servo data is written, again appropriately phase aligned with the previous track, followed by the next portion of clock data, and so on.
In such self-clocking methods, as mentioned above, it is necessary to read clock data from a previously written track whilst writing, in an interleaved manner, clock data to the next (present) track. Modern disk drives currently use a magnetoresistive read element in tandem with a conventional thin film write element in which the read and write elements are offset by a fixed distance from each other. An example of such an arrangement is shown in FIG. 1 which shows schematically a magnetoresistive read element 1 and an associated write element 2. The centres of the read and write elements 1, 2 are separated by an offset x and the read and write elements 1, 2 are shown located over a track 3 on a disk. Typically, x may be in the range 1 to 3 xcexcm and the width of the write element 2 is 1.8 xcexcm. Where the read and write elements 1, 2 are aligned with the direction of the track 3 as shown in FIG. 1, there is a write-to-read delay which depends on the rotational speed xcexd and also on the distance of the track 3 from the centre of the disk. For example, if the rotational speed xcexd of the disk is 5400 rpm and the offset x between the read and write elements 1, 2 is 1 xcexcm, then for a track at a 20 mm radius, the write-to-read delay is approximately 90 ns, and for a track at a radius of 46 mm, the write-to-read delay is approximately 40 ns. This delay, which as shown varies according to radial position across the disk, has to be compensated for during the clock track copying process.
Referring to FIG. 2, there is shown a disk 4 rotating about a centre O. The arm 5 which carries the read and write elements 1, 2 is mounted for pivotal movement about a pivot point P which is displaced from the centre of rotation O of the disk 4. This arrangement allows the arm 5 to be rigidly mounted and allows the disk to be rotated with a constant angular velocity. As can be appreciated from a study of FIG. 2, the pivotal mounting of the arm 5 about a pivot point P which is displaced from the centre of rotation O of the disk 4 means that the arm 5 will not be tangential to the disk 4 for most of the pivotal travel xcex1 of the arm 5. Instead, the arm 5 and therefore the head containing the read and write elements 1, 2 will be skewed relative to the tracks on the disk 4. Skew of the head containing the read and write elements 1, 2 relative to the tracks on the disk 4 causes a degradation of the signal read back from a clock track on the disk 4 and can make it very difficult or impossible to read back a previously written track with the read element 2 and then immediately write the next track with the write element 1. One theoretical solution to this would be to jog the read/write head between successive read and write steps but, at the frequency with which the clock tracks are written in order to obtain high throughput during manufacturing, this is technically not feasible nor practical. It is therefore desirable to minimise the amount of skew of the head during the self-servowriting process.
According to a first aspect of the present invention, there is provided a method of writing clock data to a storage medium, the storage medium having a data storage region having first and second boundaries, the method comprising the steps of: writing a first clock track to the data storage region at a first position between the first and second boundaries; writing a first set of further clock tracks to other positions on the data storage region, said other positions for the first set of further clock tracks lying between the first position and the first boundary; and, writing a second set of further clock tracks to other positions on the data storage region, said other positions for the second set of further clock tracks lying between the first position and the second boundary.
This xe2x80x9cdual-passxe2x80x9d method for writing the clock tracks to the storage medium allows the clock tracks to be written quickly with the minimum of compensation being required to compensate for movement of the write element relative to the storage medium.
Preferably, each of the first set of further clock tracks is generated from data obtained by reading clock data from a previous clock track and wherein each of the second set of further clock tracks is generated from data obtained by reading clock data from a previous clock track. In this embodiment, the signal read back from the storage medium always has a relatively large amplitude.
In a preferred embodiment, the tracks are read and written respectively with a read element and a write element that are offset relative to each other and which are skewed relative to the storage medium by a skew angle which varies according to position of the read and write elements relative to the storage medium, said first position being where the skew angle is substantially a minimum.
Thus, in the preferred embodiment, the present invention uses a dual pass self-servowrite process starting at a position where the skew angle of the read and write elements to the storage medium is a minimum (preferably zero) and writing clock tracks first to one boundary of the data storage region, returning to the start position, and then writing clock tracks to the other boundary of the data storage region. In this way, the skew angle always aids the clock recovery (i.e. the reading back of clock data) as the read back signal is enhanced from a minimum of say half amplitude at the start position to full amplitude at each boundary. This ensures that the quality and level of the clock data read back by the read element 1 are each as high as possible at all times. The read element will normally always be over known data and not random data. In contrast, writing in only one direction would cause the skew angle to work against clock recovery as the amplitude, at least towards the end of clock writing, would be less than half and probably less than is feasible to recover, and the read element may be positioned some of the time over random data on the disk.
Typically, the storage medium is a rotating disk,
Each of the first set of further clock tracks is preferably successively closer to the first boundary.
Each of the second set of further clock tracks is preferably successively closer to the second boundary.
The method may comprise the step of writing servo data to the storage medium interleaved with the clock data. Such servo data is used to locate the read and write elements of the fully assembled and preconditioned storage medium in use by an end user.
The method may comprise the step of writing location data to the storage medium and comprising the step of reading the location data to locate the position where writing of the second set of further clock tracks is to be initiated. This enables the entire set of clock tracks across the storage medium to be properly and accurately aligned with each other to provide a contiguous clock track across the whole of the data storage region of the storage medium.
According to a second aspect of the present invention, there is provided a method of writing clock data to tracks on a storage medium using a read element and a write element that are offset relative to each other and which are skewed relative to the storage medium by a skew angle which varies according to position of the read and write elements relative to the storage medium, the storage medium having a data storage region having first and second boundaries, the method comprising the steps of: (A) writing first clock data to a first clock track on the storage medium at a position at which the skew angle is substantially a minimum using the write element; (B) reading the first clock data using the read element; (C) generating clock data for a subsequent track from data obtained by reading the first clock data; (D) writing said generated clock data to said subsequent track using the write element, said subsequent track being closer to the first boundary than said first clock track; (E) writing clock data to further tracks using the write element, each of said further tracks being successively closer to the first boundary, the clock data for each of said further tracks being generated from data obtained by reading the clock data of a previous track; (F) moving the read and write elements substantially to said position at which the skew angle is substantially a minimum; (G) writing clock data to a track using the write element, said track being closer to the second boundary than said first clock track; and, (H) writing clock data to further tracks using the write element, each of said further tracks being successively closer to the second boundary, the clock data for each of said further tracks being generated from data obtained by reading the clock data of a previous track.
The method may comprise the step of writing servo data to the storage medium interleaved with the clock data.
The method preferably comprises the step of writing location data to the storage medium and comprising the step of reading the location data to locate said position at which the skew angle is substantially a minimum to enable the read and write elements to be moved substantially to said position at which the skew angle is substantially a minimum.
According to a third aspect of the present invention, there is provided apparatus for writing clock data to a storage medium, the apparatus comprising: a storage medium which has a data storage region having first and second boundaries to which clock data is to be written; a read element and a write element that are offset relative to each other and which are skewed relative to the storage medium by a skew angle which varies according to position of the read and write elements relative to the storage medium; a controller arranged to cause the write element to write first clock data to a first clock track on the storage medium at a position at which the skew angle is substantially a minimum; a clock data generator for generating clock data for a subsequent track from data obtained by reading the first clock data using the read element; a controller arranged to cause said generated clock data to be written to said subsequent track using the write element such that said subsequent track is closer to the first boundary than said first clock track; a controller arranged to cause clock data to be written to further tracks using the write element such that each of said further tracks is successively closer to the first boundary, the clock data for each of said further tracks being generated from data obtained by reading the clock data of a previous track; a controller arranged to cause the read and write elements to be moved substantially to said position at which the skew angle is substantially a minimum; a controller arranged to cause clock data to be written to a track using the write element such that said track is closer to the second boundary than said first clock track; and, a controller arranged to cause clock data to be written to further tracks using the write element such that each of said further tracks is successively closer to the second boundary, the clock data for each of said further tracks being generated from data obtained by reading the clock data of a previous track.
The apparatus may include a controller arranged to cause writing of servo data to the storage medium interleaved with the clock data.
The apparatus preferably includes a controller arranged to cause location data to be written to the storage medium and to cause the location data to be read to locate said position at which the skew angle is substantially a minimum to enable the read and write elements to be moved substantially to said position at which the skew angle is substantially a minimum.
It will be appreciated that the controllers may be embodied as a single controller and that the or each controller may be embodied in discrete logic circuits, in one or more application-specific integrated circuits (ASICs), and/or software running on appropriate programmable apparatus.